Luminaires, i.e., illumination devices may include a panel-like light guide made from optically transmissive material. One such device proposed uses a plurality of prisms formed on the light guide plate to change the orientation of the light entering thereinto from a light source. Thereby the device is capable of broadening the area that can be illuminated (for example, see Patent Document 1).
For instance, Patent Document 1 discloses an illumination device where the rear surface of the light guide plate includes a plurality of optical path changing elements. The optical path changing elements modify the optical path of the light entering the light guide plate, and cause the light to exit from the light guide plate. Each of the optical path changing elements reflect the light entering the light guide to give the light a certain directivity. The plurality of optical path changing elements include a plurality of kinds of optical path changing elements. Light reflected from at least one kind of optical path changing element is oriented in one direction, and light reflected from another kind of optical path changing element is oriented in another different direction.